1. Field of the Invention
This invention relates to the field of environmental sensors and particularly, optical environmental sensors.
2. Discussion of Related Art
Zero-order diffractive color filter (ZOF), also known as Zero Order Device (ZOD) or resonant grating or guided mode resonant filter, are made by diffractive gratings, such as parallel or crossed gratings, having a depth t and a period Λ usually smaller than a wavelength of light for which the filter is designed or isotropic microstructures with a short range ordering but long range disordering surrounded with material of lower refractive indices than the diffractive gratings.
As schematically shown in more detail in FIG. 1A, a ZOF 100 includes at least one grated structure 110 surrounded by a material 120, such as air, having a low-index of refraction (nlow) compared to grated structure 110 having a high-index of refraction (nhigh), wherein e.g., nhigh>nlow+0.2. In order for strong zero-order diffraction to take place, additional parameters have to be adjusted including: the grating period (Λ); the grating depth (t); the waveguide layer thickness (c); the fill factor or duty cycle and the grating profile or shape. As configured, grated structure 110 may act as a grated waveguide layer 130. ZOF 100 illuminated by polarized or unpolarized polychromatic light 50 (FIG. 1B) transmit and/or reflect principally the zero order of the light incident on ZOF 100. Accordingly, grated waveguide layer 130 acts like a color filter by reflection and/or transmission, because the zero order diffraction of incident light is clearly separated from higher diffraction orders. More specifically, a part of light incident onto ZOF 100 is diffracted, whereby diffraction orders higher than 0 are coupled by evanescent diffracting order into grated high-index waveguide layer 130 and then rediffracted out, whereas the remainder of the incidence light, i.e., diffraction order of 0, is transmitted through and/or reflected from ZOF 100 into light 52 and light 51, respectively. The coupling of light into grated waveguide layer 130 occurs at certain wavelengths and at a certain angular orientation φ where resonance causes destructive interference. Thus, ZOF 100 possess characteristic reflection and/or transmission spectra depending on a viewing angle Θ and the orientation φ of the grating lines with respect to the observer, not shown.
For each pair of angles they directly reflect a particular spectral range or color. As long as the materials used possess no absorption the transmission spectra are the complement of those in reflection. For example, as schematically shown in FIG. 1C and FIG. 1D, the reflected and/or transmitted light 51 and 52 may be of the blue and yellow wavelength, respectively. In contrary to higher order diffraction devices, for a ZOF such as ZOF 100, light is reflected at a viewing angle which is equal to the incidence angle. Documents related to color filters based on ZOF are discussed in the following paragraphs.
In U.S. Pat. No. 4,484,797 to Knop et al. a variable index-of-refraction optical medium of certain minimum thickness and periodicity with respect to the wavelength of incident light is disclosed, if it meets certain specified constraints with respect to (1) relative indices-of-refraction of both its internal structure and that of its surroundings and (2) relative values of incident wavelength to periodicity and the relative indices-of-refraction-operates to produce both angularly-dependent subtractive-color filter reflection spectra and subtractive-color filter transmission spectra in accordance with its physical parameters.
FIG. 2A, FIG. 2B and FIG. 2C schematically illustrate various possible profiles for grated structures 110, namely rectangular, generally sinusoidal and triangle shaped profiles, respectively.
WO2006/038120 to Walter discloses a security device including a first zero order diffractive microstructure on a substrate, a second zero order diffractive microstructure, and an intermediate light transmissive layer separating the two diffractive microstructures. The disclosed security device allows interference of visible light to take place.
EP1882961 to Walter et al. discloses a Zero-order diffractive filter including a first layer with a periodic diffractive microstructure, forming a waveguide, and at least one adjacent second layer, wherein the first layer has a refractive index that is higher than the refractive index of the second layer by at least 0.2. At least one of the second layers is a porous layer including nanopores. The period of the diffractive microstructure is between 100 nm and 3000 nm.
EP1862827 to Stuck et al. discloses a zero order diffractive filter for polarized or unpolarized polychromatic light, including a grating line microstructure formed by a surrounding medium with a low index of refraction n low and a waveguide layer with a high index of refraction n high, the grating lines having a grating period Λ that is smaller than the wavelength of light for which the filter is designed. A plurality of single nanostructures with dimensions in the nanometer range is superposed on a first interface between the surrounding medium and the waveguide layer. In one embodiment, the zero-order diffractive filter includes an interface between a low refraction index medium and the waveguiding layer that is nanostructured.
WO03/064995 to Cunningham et al. describes a label-free guided mode resonant filter biosensor using a linear one-dimensional grating surface structure. The shift of a narrow reflection peak is measured. The peak shift is caused by a change in the index of refraction at the surface of the grating due to the interaction of biological substances with the sensor. Due to the narrowness of the reflection peak and the minor peak shift, the filter shows no visible color effect. Further these filters work in the near infra red spectral range.
EP 1990661 to Blondiaux et al. discloses an isotropic zero-order diffractive color filter, a method to manufacture an embossing tool and a method to manufacture such a filter. The zero-order diffractive color filter includes diffractive microstructures and a wave-guiding layer, wherein the diffractive microstructures possess a short range ordering over at least four times the period of the microstructures, and the diffractive microstructures possess a long range disordering over length scales of more than 100 micron.
Further details concerning zero-order diffractive filters can be found in M. T. Gale, “Zero-Order Grating Microstructures” in R. L. van Renesse, Optical Document Security, 2nd Ed., pp. 267 287. The correct choice of grating period, depth and profile with a defined number of high and low material indices transitions can make a specified color band pass filter or notch filter.